Red blood cell progenitors undergo self-renewing divisions prior to the commitment switch to erythroid differentiation but the pathways that regulate erythroid progenitor growth are still largely unknown. In a computational search for genes expressed in definitive (adult) but not primitive (embryonic) red cell lineages, we identifie the nuclear receptor transcription factor VDR, which is activated by binding to its ligand, vitamin D3 (1,25(OH)2D3). Real-time RT-PCR analysis indicated that Vdr is expressed in definitive erythroid progenitors from mouse fetal liver and bone marrow and is downregulated during erythroid maturation. Structural studies have shown that VDR activation by the vitamin D3 ligand results in significant conformational changes that stabilize the protein and induce its translocation into the nucleus, where it recruits coregulatory complexes. The VDR signaling pathway has been studied mostly in bone. The regulation of erythropoiesis by this pathway has been essentially unexplored; published studies were performed almost entirely in leukemic cell lines (not normal primary cells). We find that vitamin D3 stimulates the growth of erythroid progenitors (BFU-E and CFU-E) from mouse fetal liver and bone marrow. Not only the numbers but also the size of the BFU-E colonies is increased when the VDR pathway is activated. The CD71low subset of c-Kit+ fetal liver progenitors, which contains BFU-E, is the most sensitive to activation of VDR. Progenitors cultured with vitamin D3 differentiate normally. Vitamin D3 can partially substitute for dexamethasone (a glucocorticoid) in progenitor cultures, suggesting a possible role in stress erythropoiesis. We hypothesized that VDR activates or represses genes in developing erythroid progenitors in combination with erythroid transcription factors (TFs) such as Gata, Klf1/Eklf, and Scl/tal and that DNA binding sites for VDR may cluster with binding sites for erythroid TFs. A computational approach based on this hypothesis was used to identify candidate VDR target genes in erythroid progenitors. We have begun to confirm the vitamin D3 responsiveness of some of these genes using real-time RT-PCR. The candidates include genes known to function in erythropoiesis (c-Myc, Gata2) and others that were not (Calcium binding kinase Camk1d; N-Myc; Mlx-interacting protein, a bHLH transcription factor; Grtp1, GTPase Rab activator). We hypothesize that the VDR pathway controls erythroid progenitor cell proliferation and/or survival, at least in part through transcriptional regulation of target genes. The goal of this project is to elucidate the molecular mechanisms by which vitamin D regulates red blood cell development.